"Huckleberry Finn", for sure. It's a masterpiece. So too "Tom Sawyer".   Walt Whitman's "Leaves of Grass". The series of "private eye" adventures written by Raymond Chandler about the fictional private eye, Philip Marlowe. A number of Louis L'Amour's best adventure stories such as, for one, "Jubal Sackett".

By the way, Mark Twain's greatest book, in his OWN opinion, and in mine also is:

"Joan of Arc"

(Though it is not a book that in any way relates to Americana, it is a great book written by a great American author, and it's a shame that it's not better known, as I suspect it would be had it been written about an American life, instead of the life of a French peasant girl and a saint from the 1400s. In Mark Twain's day, France was still gratefully remembered by Americans as the courageous ally which had assisted the American revolutionaries in attaining independence from the British Empire. Americans' view of France then was radically different from the common view of France now. The French were seen then as a heroic nation both militarily and in terms of their political and social ideals.)